The sleep/wake cycle of older adults tends to become advanced relative to the rest of the world. This can be a difficult and frustrating experience, since the circadian rhythms are not in synchrony with the environment. Exercise is a stimulus capable of phase-delaying the circadian rhythms of younger people. The purpose of this study is to 1) examine whether a 3-hour bout of low-intensity nocturnal exercise is capable of phase-delaying the circadian rhythms of both young and older adults; 2) determine the immediate effects of nocturnal exercise on hormonal secretions; and 3) determine how closely different markers of circadian phase correspond to each other. Subjects will be admitted to the General Clinical Research Center (GCRC) and will be studied under two conditions in a within-subjects design. The exercise condition will involve a 3-hour, low-intensity, nocturnal exercise bout (5 consecutive 36-minute exercise bouts with 15 minutes treadmill exercise at 40% of peak VO2 followed by 15 minutes of treadmill exercise at 60% of peak V)2, followed by a 6-minute rest) that is tailored to each individual. Subjects will remain sedentary during the control condition and will have the same sleep/wake cycle as in the exercise condition. To assess circadian rhythms, hormonal secretions in plasma will be measured. Phase shifts in the exercise condition will be compared with phase shifts in the control condition. We predict that nocturnal exercise will phase-shift the circadian rhythms of older adults as much as in young subjects, that exercise will increase the secretion of certain hormones, and that the different markers of circadian phase will be highly correlated. This study will advance the understanding of the effectiveness of treatments for sleep disturbances in older adults.